1. Technical Field
The present invention relates a semiconductor laser device with a dielectric protective film formed on the end surface of a semiconductor layer using a nitride semiconductor, and more particularly to a high power semiconductor laser device using a nitride semiconductor substrate. A III-V group nitride semiconductor including GaN, AlN and InN, and a mixture crystal of them as AlGaN group, InGaN group and AlInGaN group can be given as the specific composition of the semiconductor device.
2. Description of Related Art
Nitride semiconductor elements have light emission from the ultraviolet range in relatively short wavelength to the visible light range comprising red, and are widely used as materials that compose a semiconductor laser diode (LD), a light emitting diode (LED), and so on. In these years, they are getting to improve the miniaturization, life, reliability and output power, and are used mainly for light sources in electronic devices, such as personal computer and DVD for electronics device, medical devices, machining devices, the optical fiber communications, and so on.
These type of nitride semiconductor elements have a laminated structure mainly having a buffer layer, an n-type contact layer, a crack prevention layer, an n-type cladding layer, an n-type light guide layer, an active layer, a p-type electron confinement layer, a p-type light guide layer, a p-type cladding layer and a p-type contact layer that are successively laminated on a sapphire substrate. In addition, a stripe-shaped ridge is formed by etching, or a current narrowing layer is formed to provide a stripe-shaped waveguide region. The n-type and p-type contact layers are provided with n-side and p-side electrodes, respectively. A current is applied through the electrodes, thus, the active layer emits light. Resonance surfaces are formed at a prescribed resonance length in the both end surfaces of the waveguide region. Laser light is emitted from the resonance surface.
Insulating protective films, or the like, are formed on the resonance surfaces to protect the semiconductor layers from air and to provide the reflectivity difference between the emission side and the rear side. The protective film on the rear side has a high reflectivity as compared with the protective film on the emission side. This can improve the output power.
In semiconductor laser device having protective films with large reflectivity difference between the rear side and the emission side, light that leaks from the waveguide region (stray light) is not prone to outgo from the rear side, and thus is emitted from the end surface on the emission side. Accordingly, the stray light may cause noise in the far-field pattern (FFP), and, as a result, non-Gaussian distribution. In order to prevent outward emission of the stray light, a non-transparent film, such as metal film, can be formed so as to cover the end surface of the substrate. (See Patent Document 1: Japanese Patent Laid-Open Publication TOKUKAI No. 2002-280663)
However, in the case where a non-transparent film is partially formed on the resonance surface on the light emission side, an additional process, such as mask-formation process, is required. Particularly, in the case where, after a wafer is divided into bar-shaped laser elements, an end surface protective film is formed on the end surface of the bar-shaped laser element, it is difficult even to form a mask in precise alignment. For this reason, it is more difficult to control the region where the end surface protective film should be formed. Particularly, in the case where a metal material is used as a non-transparent film, if the alignment control is poor, there is a problem that a short circuit may occur. In addition, in the case where a non-transparent film is formed on a large region, the bonding characteristics between the non-transparent film and the semiconductor layer or other protective film decreases due to the difference of the thermal expansion coefficient between the metal material and the semiconductor layer in the case of some materials. In this case, a problem that the non-transparent film is prone to peel off arises.